We simply use a pallet with a solid base or close up the gaps in the pallet to prevent damage to the firework.
Best not to use a pallet without filling in the gaps, as the tubes can blow through the gaps and render the firework useless.
Provides a very nice flat surface, we then use 4 bricks around the firework we are firing at the time.
Just beyond the pallet is one of them funky ladders that open in all directions, ours has metal plates to walk along, which we place a few pots on filled with sand for the smaller bits and bobs
Finally on the one side of the ladder, we use a bit of sleeper, upright, which is attached by a ratchet strap for any wheels we have